1. Field of the Invention
The present invention relates to an IP telephone system, an IP telephone apparatus and a calling method via an IP network.
2. Description of Related Art
Recently, the Internet has rapidly spread, and then an IP telephone apparatus that allows voice communication via an IP network (hereafter referred to as IP telephone) has rapidly grown as well. However, the rapid growth of IP telephones causes a problem that no communication can be established although a media stream is established. Thus, it has been proposed to create a technology which avoids this problem (see Prior Art 1). The proposal is to solve problems which can occur because an IP network is congested. While the IP network is congested, CA (Call Agent) on an IP network inevitably bears a heavier workload. Therefore, it is currently requested to reduce the workload for CA on an IP network.
FIG. 9 illustrates a communication network used for a conventional IP telephone system. In the following, a communication network between IP telephone A901 and IP telephone B902 shown in FIG. 9 is described.
In FIG. 9, IP telephone A901 and IP telephone B902 are using different providers and managed by CAs which are owned by different carriers. IP telephone A901 and IP telephone B902 have functions that enable voice communication via an IP network and a Public Switched Telephone Network (hereafter referred to as PSTN). CA also has SIP (Session Initiation Protocol) server functions.
When IP telephone A901 communicates with IP telephone B902, communication is established via the IP network or the PSTN.
When communicating via the IP network, IP telephone A901 transmits, to CA 904 owned by carrier A, the message “INVITE” which includes information regarding the destination terminal (IP telephone B902) via modem A 903. CA 904 searches for another CA that manages the destination terminal, since CA 904 does not manage the destination terminal. As a result of the search, CA 905 that is owned by carrier B is found, and the aforesaid message “INVITE” is transmitted from CA 904 to CA 905. CA 905 transmits the message “INVITE” to IP telephone B902. IP telephone B902 receives the message “INVITE” via modem B 906. When IP telephone B902 receives the message “INVITE”, messages specified by SIP are exchanged between IP telephone A901 and IP telephone B902. Then, the two telephones are ready to communicate.
On the other hand, when communicating via the PSTN, IP telephone A901 inputs the telephone number of IP telephone B902 and places a call to IP telephone B902. Then, IP telephone B902 receives a call from IP telephone A901 via the PSTN. When the receiver of IP telephone B902 is picked up, IP telephone A901 and IP telephone B902 are connected, and the two telephones are ready to communicate.
FIG. 9 also shows a communication network between IP telephone A901 and a terminal apparatus on a cellular phone network or the PSTN. In this example, voice date received from IP telephone A901 is converted, at MG (Media Gateway) 907, into a data format necessary for communication via the PSTN or the cellular phone network. Then, the converted data is transmitted to the PSTN or the cellular phone network via switchboard 908.
When communication is attempted via the IP network and when there are some problems occurred within the IP network, CA 904 establishes communication via networks other than the IP network. For example, when communication is attempted via the PSTN, voice data received from IP telephone A901 is transmitted to the PSTN via MG 907 and switchboard 908 as described above. Then, voice data is transmitted to IP telephone B902 via switchboard 909 and MG 910 of carrier B.
FIG. 10 is a sequence diagram to illustrate a communication network via the IP network.
When IP telephone A901 places a call, the message “INVITE” is transmitted to CA 904. CA 904 transmits the message “INVITE” to CA 905. At the same time, CA 904 transmits the message “100 trying” to IP telephone A901. Similarly, CA 905 transmits the message “INVITE” to IP telephone B902. At the same time, CA 905 transmits the message “100 trying” to CA 904.
Upon receiving the message “INVITE”, IP telephone B902 transmits, to CA 905, the message “100 trying” and then the message “180 Ringing”. At the same time as transmitting the message “180 Ringing”, IP telephone B902 sounds a receiving tone. CA 905 transmits the message “180 Ringing” to CA 904. CA 904 transmits the message “180 Ringing” to IP telephone A901. IP telephone A901 then sounds a ringing tone in response to the message “180 Ringing”.
When it is determined, through the receiving tone, that the receiver of IP telephone B902 is picked up, IP telephone B902 transmits, to CA 905, the message “200 OK” to indicate a connection approval. CA 905 transmits the message “200 OK” to CA 904. CA 904 transmits the message “200 OK” to IP telephone A901.
Upon receiving the message “200 OK”, IP telephone A901 transmits the message “Acknowledge (hereafter referred to as ACK)” to CA 904. CA 904 transmits the message “ACK” to CA 905. CA 905 transmits the message “ACK” to IP telephone B902. When IP telephone B902 receives the message “ACK”, IP telephone A901 and IP telephone B902 are ready to communicate via the IP network.
[Related Art 1]
Japanese Patent Laid Open Publication 2003-333106
However, when communicating via the IP network as described above, CA needs to relay all the messages, such as “INVITE” or “200 OK”, to destination terminals or other CAs. This puts an excessive workload on CA and will thus become a more serious problem in the future with the growth of IP telephones.
Also, when communication is attempted via the PSTN and when the calling terminal does not store information regarding whether communication can be established with the destination terminal via the PSTN, predetermined signals (e.g., SIP “INVITE” message) are transmitted to CA when calls are placed. Thus, CA handles all the signals transmitted via the PSTN as well as the IP network, which puts an excessive workload on CA.
The above-described problems can be solved by improving CA's process capacities. However, this requires significant costs, and the users may have to bear the cost. This can lead to hinder the diffusion of IP telephone systems.